Nonlinear optical devices

ABSTRACT

Optically nonlinear device elements such as directional couplers, switches, frequency stabilizers, optical parametric devices and modulators use as an optically nonlinear element a cross-linked triazine polymer containing a covalently bonded optically nonlinear dye moiety. A specific cross-linked triazine with this dye moiety may be made by cyclotrimerizing a p-(N,N-bis(4&#39;-cyanatobenzyl)amino)-p&#39;-(2,2-dicyanovinyl)azobenzene monomer. During polycyclotrimmerization or cure, the element is subjected to a poling voltage which aligns the dipoles of the dye moiety to give a large useful nonlinear susceptibility.

This is a division of application Ser. No. 525,947 filed May 18, 1990now U.S. Pat. No. 5,045,364.

TECHNICAL FIELD

This invention relates to nonlinear optical devices such as electroopticmodulators and switches, frequency converters, data processors, opticalparametric oscillators and amplifiers, and optically nonlinear materialsuseful in such devices.

BACKGROUND OF THE INVENTION

Optical transmission systems have come into widespread use primarilybecause of the ability of optical fibers to transmit much greaterquantities of information than other comparable transmission media.Processing of such information normally requires that the information beconverted to an electronic form. Thus, it has long been realized that ifsuch functions as modulation, switching, mixing, data processing and thelike could be performed directly on lightwaves, optical communicationssystems could be made to be much more efficient. It is also known thatoptically nonlinear materials can be used to make electroopticalmodulators, switches, optical parametric devices and other devices foroperating directly on lightwaves. Lithium niobate is the most commonlyused nonlinear medium, although certain organic crystalline materialshave also been proposed.

The patent of Dirk et al., U.S. Pat. No. 4,859,876, granted Aug. 22,1989, hereby incorporated herein by reference, describes a nonlinearelement comprising a glassy polymer containing an optically nonlinearorganic moiety. The nonlinearity results from electric poling whichaligns permanently dipoles within the polymer. The glassy polymer thatwas principally described was polymethylmethacrylate (PMMA), while otheracrylate based polymers were also mentioned. The Dirk et al. patentrepresents a significant advance of the state of the art since polymerssuch as PMMA can be applied as a film to a substrate and theirproperties controlled much more easily and accurately than crystallinesubstances. The PMMA films constituting the heart of the variouselectrooptic devices may range from only about one micron to about twohundred microns in thickness.

A problem with the nonlinear devices of Dirk et al. is that theirnonlinear susceptibility tends to deteriorate over time, particularlywhen subjected to high temperatures on the order of or exceeding 80° C.Such lower susceptibilities generally mean that the devices perform thefunctions for which they were intended with less efficiency than wouldotherwise be the case. As a consequence, systems which use these devicesmay require special cooling apparatus to keep the devices from reachingelevated temperatures and other design precautions may be required tocompensate for a deterioration of optical properties with time.

SUMMARY OF THE INVENTION

In accordance with the invention, an optically nonlinear elementcomprises cross-linked triazine polymer containing a covalently bondedoptically nonlinear dye moiety. Cured triazine strongly stabilizes thisdye moiety, and such stabilization continues over time and underconditions of high temperature. This stability also characterizes thedipole alignment needed for high nonlinear susceptibility. The triazineoffers the advantages of the materials described in the Dirk et al.patent, such as ease of use in a thin film form, and yet is inherentlythermally stable. As will be described in detail, triazine polymer canbe made with a dicyanovinylazo moiety that is capable of maintaining alarge nonlinear susceptibility while being transparent over a usefuloptical wavelength of about 0.8 to about 2.0 microns. A specificcross-linked triazine with this dye moiety may be made bycyclotrimerizing ap-(N,N-bis(4'-cyanatobenzyl)amino)-p'-(2,2-dicyanovinyl)azobenzenemonomer. During the cyclotrimerization or cure, the element is subjectedto a poling voltage which aligns the dipoles of the dye moiety to give alarge useful nonlinear susceptibility.

These and other objects, features and advantages of the invention willbe better understood from a consideration of the following detaileddescription taken in conjunction with the accompany drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic perspective view of an electrooptical directionalcoupler or switch in accordance with an illustrative embodiment of theinvention;

FIG. 2 is a schematic perspective view of an integrated solid statelaser tuner and frequency stabilizer, within which the invention isused;

FIG. 3 is a diagrammatic representation of a device for generatingsecond harmonic frequencies, within which the invention is used;

FIG. 4 is a diagrammatic representation of an electroopticphase/intensity modulator within which the invention is used;

FIG. 5 is a perspective schematic view of an electrooptical guided waveintensity modulator within which the invention is used;

FIG. 6 is a diagrammatic illustration of a method for making a syntheticmonomer in accordance with one feature of the invention;

FIG. 7 is a diagrammatic illustration of a method for making triazinepolymer from the monomer of FIG. 6;

FIG. 8 is a graph comparing the thermal stability of two opticallynonlinear materials of the prior art with that of a triazine materialmade in accordance with the invention; and

FIG. 9 is a diagrammatic illustration of a generalized monomer that canbe used for making triazine in accordance with one aspect of theinvention.

DETAILED DESCRIPTION

The devices described herein operate directly on light waves andillustrate different device functions. These devices make use of apolymer, specifically, triazine, having within it a constituent that hasa finite nonlinear optical susceptibility. This nonlinear opticalcharacteristic is imposed by the known process of electric poling, inwhich an electric field is applied to the material to align dipoles ofconstituent molecules permanently in generally the same direction duringcure. The devices of FIGS. 1-5 demonstrate various ways in which thenonlinear optical susceptibility of such a nonlinear polymeric elementcan be exploited. In all of these devices, it is intended that thenonlinear material be substantially transparent at the opticalwavelength of operation; that is, its attenuation at such wavelength issufficiently low so as to allow for a commercially operable and feasibledevice.

Referring to FIG. 1, there is shown schematically an electroopticdirectional coupler or switch 1. The coupler comprises a substrate 2, apair of spaced channel waveguides 3 and 4 on one surface of thesubstrate, and a pair of electrodes 5 and 6, one electrode associatedwith and contiguous to each of the channel waveguides. At a centralregions 7 and 8 of the waveguides 3 and 4, respectively, the waveguidesare parallel to each other and the spacing between them is small,typically from five to twenty microns. The electrodes 5 and 6 arepositioned adjacent the waveguides in this narrowly spaced region so asto maximize the electric field developed across the waveguides uponapplication of a voltage to the electrodes. Alternatively, as shown bythe dotted lines, one may employ top and bottom electrodes on oppositesides of the waveguides to enhance the field for a given appliedvoltage.

The waveguides 3 and 4 are made of a triazine material exhibiting anonlinear optical susceptibility in response to an applied electricfield. Light, preferably from a laser, is directed into one end ofwaveguide 3 as designated by S₁. In the absence of any applied field,the electromagnetic field associated with the light extends beyond theconfines of waveguide 3 and penetrates waveguide 4 in region 8 of thatwaveguide. If the length of the portions 7 and 8 are properly selected,the light will essentially be emitted from waveguide 4 as shown by S₂.There is thus a complete transfer of the light from one waveguide to theother. By applying an appropriate voltage by means of the electrodes,the nonlinear response of the waveguide material to the electric fieldcan produce a slight change in the transmission characteristic of thewaveguide. When properly adjusted, this voltage or electric field willprevent the transfer of light from one waveguide to the other and thelight will be transmitted directly through waveguide 3 so as to beemitted as shown by S₃. The nonlinear optical susceptibility is oftendesignated by χ, whose value is a designation of the efficiency of theelement; that is, the higher the value of χ the greater the response toan applied electric field. When an optically nonlinear material respondsto a voltage or an electric field in this manner, it is often referredto as electrooptic material.

FIG. 2 shows an integrated laser tuner and frequency stabilizer usingthe triazine nonlinear or electrooptic medium described herein. Ajunction or injection laser 20 is devised such as to have a lightemitting junction 23 which is contiguous to a thin electrooptic film.The film 22 may be provided with opposing electrodes 24 and 25 as shownon one side of the laser, or by electrodes 24a and 25a as shown on theother side of the laser. Either electrode configuration can be used toexcite an optically nonlinear response in the nonlinear film 22. Theelectrooptical effect of the film under properly applied fields resultsin a useful tunable filter function, or as a frequency stabilizer of thelaser output.

FIG. 3 shows the triazine optically nonlinear medium used in an opticalparametric device for second harmonic frequency generation. Here anincident light beam 30 at a frequency f impinges on the opticallynonlinear film 31. Due to the nonlinear optical properties, two colinearbeams 32 and 33 are emitted, one at the same frequency f and one attwice the original frequency, 2 f. The emerging colinear beams 32 and 33may be directed through a prism 34 which spatially separates them intoseparate beams 32 and 33. The beam of frequency 2 f may be usedindependently of the other beam if for any of various reasons a higher 2f frequency is desirable. This embodiment demonstrates that an appliedelectric field is not always required for the nonlinear optical elementto preform a useful function.

FIG. 4 shows the use of a triazine nonlinear polymer film in anelectrooptical phase/intensity modulator. Here, incident light 40 havinga polarization P₁ is passed through a nonlinear film 41, which isprovided with transparent electrodes 42 and 43 on opposite surfaces.Upon passing through the film, the natural birefringence of theoptically nonlinear film causes a change of polarization of the light toP₂. When a voltage is applied to the film by means of a voltage source44, the electric field applied to the film changes its opticalproperties. The nonlinear response causes a change in the film's indexof refraction, thereby altering its birefringence and this, in turn,causes the emitted light to have yet a different polarization designatedas P₃. Thus, a polarization modulation between the values P₂ and P₃ canbe achieved through periodic applications of the voltage to the film 41.An intensity modulation can optionally be obtained by placing apolarizer 45 at the output path of the beam which is oriented to allowthe passage of either P₂ or P₃, but not both. It should be noted thatthe nonlinear optical film of both devices of FIGS. 3 and 4 arepreferably deposited on a transparent substrate.

FIG. 5 shows a guided wave electrooptic intensity modulator employing atriazine nonlinear optical layer 50. The film 50 is formed on aconductive substrate 51 having an insulative coating 52. The film isformed as an interferometric waveguide structure with an electrode 53placed on one arm 54 of the interferometer. As voltages are applied toarm 54 by electrode 53, an electric field is produced in arm 54 of theinterferometer. This field changes the index of refraction of thematerial and results in an effective change of the optical path lengthin arm 54 of the interferometer relative to the other arm. This in turnproduces either constructive or destructive interference of light at arecombination point 55. As the voltage is modulated so as to alternatebetween constructive and destructive conditions, the output intensityvaries between maximum and minimum values as well.

The nonlinear optical medium used in all of the devices of FIGS. 1-5 andother electrooptic and optical parametric devices that may be made inaccordance with the invention, comprises cross-linked triazinecontaining a covalently bonded optically nonlinear dye moiety. Inaccordance with one feature of the invention, the dye moiety may bedicyanovinylazo dye, which is substantially transparent to light havingwavelengths between 0.8 and 2.0 microns and, accordingly, the light withwhich such devices are used should be of a corresponding wavelength. Thedipoles of the triazine molecules are aligned by applying a polingvoltage during cure (i.e., during cross-linking), as will be explainedmore fully later. A detailed method that I have used in the laboratoryfor making triazine with a dicyanovinylazo moiety will now be discussed.

FIG. 6 summarizes a method for making a synthetic monomer in accordancewith the invention while FIG. 7 shows the method for making triazineoligomers from the synthesized monomer; the final polymer results fromcuring the oligomers. Referring to FIG. 6, 60 refers to a startingmaterial; 7.0 grams of thionyl chloride (SOCl₂) was added drop by dropinto a solution of material 60 in sixty milliliters of CH₃ CN. Thisyielded an intermediate 61 which was separated from a gummy residue bydraining from a separatory funnel. The intermediate 61 was added overten minutes into a well stirred solution at room temperature of 2.36grams of aniline (PhNH₂) in twelve grams of triethylamine (TEA). Afterfiltering off triethylammonium hydrochloride, the solution wasconcentrated to yield a syrup of the intermediate compound shown in FIG.6 as 62, which may be designated as bis(4-hydroxybenzyl)aniline. Sevengrams of the intermediate 62 was dissolved in fifteen milliliters ofacetone and the solution cooled in a xylene liquid nitrogen bath to -15°C. To this was added a solution of cyanogen bromide (CNBr) in acetone(3.0 grams CNBr in seven milliliters of acetone). This was followed bythe addition by drops of four grams of triethylamine. The temperaturewas maintained at about -10° C. for fifteen minutes and then warmed toroom temperature in one hour. This yielded the intermediate 63 which maybe designated as bis(4-cyanatobenzyl) aniline.

Seven hundred milligrams of intermediate 63 were dissolved in twentymilliliters of acetone. To this was added four drops of acetic acid anda solution of one thousand eighty milligrams of diazonium salt(4-(2,2,-dicyanovinyl)benzene diazonium hexafluorophosphate) in twelvemilliliters of acetone. It should be noted that the diazonium salteventually constitutes the dye moiety of the final polymer. The mixturewas stirred in a nitrogen atmosphere for eighteen hours at roomtemperature and then heated at 44° C. for one hour. The mixture was thenmixed with fifty milliliters of equal parts of acetone and water and theprecipitate collected and washed with water until neutral. When dried,this yielded 1.15 grams of intermediate 64, a monomer which may bedesignated as p-(N,N-bis(4'-cyanatobenzyl)amino)-p'-(2,2-dicyanovinyl)azobenzene.

Referring to FIG. 7, the monomer 64 (100 milligrams) was next dissolvedin a solvent such as methylethylketone (MEK) or γ-butryrolactone (500milligrams). The γ-butryrolacetone is preferred. To this solution wasadded a metal complex catalyst at a concentration of between 0.1 to fivepercent by weight of the monomer, preferably 0.8 to 2.0 percent. Typicalcatalysts are copper benzoylacetonate (CBA), zinc benzoylacetonate, andcopper or zinc naphthenate, although CBA is preferred. This solution wasthen heated in a sealed tube at 150° C. for thirty minutes to initiatecyclotrimerization. The resulting oligomer solution is appropriate forcoating as by spin-coating on a substrate such as an aluminized wafer.The coating is next polymerized or, more specifically,polycyclotrimerized, by heating at a temperature of between 100° and170° C., preferably between 130° and 160° C. The temperature ispreferably raised quickly to 100° C., and thereafter raised at about twodegrees per minute to the final temperature, and is held at the finaltemperature for about one-half hour.

The oligomer 66 contains a dicyanovinylazo dye moiety 65 which may bepoled to impart a nonlinear optical susceptibility. During the cure, anelectric field typically of 1×10⁶ volts per centimeter or more isapplied to the coating so as to pole the dye moiety. The electric fieldmay be applied between parallel electrode plates, or alternatively maybe applied by corona poling as is known in the art.

During cure, the solution was polycyclotrimerized to yield across-linked triazine polymer which has been poled to be opticallynonlinear. As is known, cross-linked triazine extends in threedimensions and therefore differs from polymers such as PMMA whichextends in only two dimensions. The geometry resulting fromthree-dimensional cross-linking is believed to confine the poled dyemoieties 65 more strongly than would be the case with two-dimensionalpolymers. As a consequence, over time and under conditions of relativelyhigh temperature, the dye moieties 65 remain firmly confined within thetriazine polymeric structure.

This advantage has been verified experimentally in tests, the results ofwhich are summarized in FIG. 8. FIG. 8 shows the variation of r forthree different materials as a function of time in days, where r is theratio of the electrooptical coefficient of the poled film to theoriginal electrooptic coefficient. Thus, if there is no change withtime, r will remain one. The electrooptic coefficient is a function ofboth the optical susceptibility (in this case, the second order opticalsusceptibility) and the density of nonlinear moieties. Thus, a decay ofr generally indicates a decay in number of effective nonlinear moieties.Curve 67 shows the change of electrooptic coefficient with respect totime of the material discussed in the Dirk et al. patent, namely,disperse red dye 1 dissolved in polymethylmethacrylate (DR1/PMMA). At25° C., room temperature, the curve 67 shows a reduction of electroopticcoefficient after ten days to a value of less than half the originalvalue. Curve 68 shows that at 80° C. there is a precipitate drop tovirtually zero, which indicates that the material could not practicallybe used at that temperature. Curve 69 is an example of a covalentlybonded nonlinear dye molecule in a glassy polymer host as described ingeneral in the Dirk et al. patent, the material beingdicyanovinylazobenzene-methylmethacrylate (DCV-MMA). Curve 69 shows thatthere is some deterioration with time at 25° C. Curve 70 again shows aprecipitate drop in electrooptic coefficient at 80° C. and thereafter acontinued deterioration with time.

Curve 71 shows the change of electrooptical coefficient of triazine at85° C. One can see that even at this high temperature, there is only amodest deterioration and that, after ten days, the second order opticalsusceptibility is about eight-tenths of its original value. The originalmeasured value of the second order optical susceptibility χ.sup.(2) wasequal to about 75×10⁻⁹ esu (electrostatic units). This demonstrates thattriazine is practical for use as a nonlinear element in the variousdevices of FIGS. 1-5, in environments that may consistently be heated totemperatures as high as 85° C., and that such reliability has anextended lifetime.

The triazine oligomer solution that we have described can be coated onany of various substrates using any of various techniques well known inthe art. After coating, the solvent is removed during the heat and cureprocess. Multiple coatings can be made, but it is believed that mostuseful device coatings will be in the thickness range of one micron totwo hundred microns. Any of various substrates can be used and a polingconductor may be provided on one surface of the substrate.

The monomer 64 of FIGS. 6 and 7 from which the triazine molecule is madeshould be considered as only one example of a suitable triazineprecursor. FIG. 9 shows a generalized formula for triazine precursorsthat could be used. In FIG. 9, R1 and R2 may be (CH₂)n or ##STR1## whichmay also be designated as 2-(4'-methylenephenylene)propylidenyl, where nis an integer from zero to ten. R3 and R4 may be hydrogen, alkyl,alkenyl, alkoxy, or aryloxy. R5 may be hydrogen, alkyl or alkoxy. R6 maybe p-(2,2-dicyanovinyl)phenyl, p-(1,2,2-tricyanovinyl)phenyl,5-(2,2-dicyanovinyl)thiazolyl, 5-(1,2,2-tricyanovinyl)thiazolyl,4-chloro-5-(2,2-dicyanovinyl)thiazolyl,4-chloro-5-(1,2,2-tricyanovinyl)thiazolyl, or 5-nitrothiazolyl.

The foregoing has shown in detail how cross-linked triazine containingan optically nonlinear dye moiety can be made and used in any of varioususeful devices. The second order susceptibility is competitive to thatof materials described in the prior art, even at elevated temperatures,but it may have useful susceptibilities at the third or higher orders aswell. Various other methods for making triazine with such dye moiety maybe made by those skilled in the art without departing from the generalteachings hereof. Devices other than those explicitly described formaking use of triazine as the optically nonlinear element may likewisebe made by those skilled in the art. Various other embodiments andmodifications of the invention may be made without departing from thespirit and scope of the invention.

I claim:
 1. An optical device comprising:an optically nonlinear elementcomprising cross-linked triazine containing a covelently bondedoptically nonlinear dye moiety; means for defining an optical input toand an optical output from said element; the optically nonlinear elementbeing substantially transparent to optical energy intended to beprovided at the optical input to the nonlinear element.
 2. An opticaldevice as recited in claim 1 further comprising;means for applying anelectric field to said nonlinear element for altering an opticalproperty thereof.
 3. The optical device of claim 1 wherein:the elementis triazine film on a substrate.
 4. The device of claim 3 wherein:thefilm is from one micron to two hundred microns in thickness.
 5. Thedevice of claim 1 wherein:the cross-linked triazine is made bycyclotrimerizingp-(N,N-bis(4'-cyanatobenzyl)amino)-p'-(2,2-dicyanovinyl)azobenzene. 6.The device of claim 5 wherein:during at least part of saidcyclotrimerization, the element is subjected to a poling voltage whichaligns dipoles of the optically nonlinear dye moiety.
 7. The device ofclaim 6 wherein:the dye moiety is dicyanovinylazo dye.
 8. The device ofclaim 7 wherein:the optical input energy intended has a wavelengthbetween about 0.8 and about 2.0 microns.
 9. The device of claim 1wherein:the cross-linked triazine is made by cyclotrimerizing themonomer of FIG. 9 hereof.
 10. The device of claim 5 wherein:thep-(N,N-bis(4'-cyanatobenzyl)amino)-p'-(2,2-dicyanovinyl)azobenzene ismade by the process of FIG. 6 hereof.
 11. The optical device of claim 1further comprising:a second nonlinear element in close proximity to theoptically nonlinear element; means for optionally transferring opticalenergy from the optically nonlinear element to the second nonlinearelement; the transferring means comprising means for optionallyestablishing an electric field in the optically nonlinear element. 12.The optical device of claim 1 further comprising:a semiconductor laserclosely optically coupled to the optically nonlinear element; and meansfor stabilizing the output frequency of the laser comprising means forapplying an electric field across the optically nonlinear element. 13.The optical device of claim 1 wherein:the optically nonlinear elementcomprises means for generating a second harmonic of light provided atthe optical input thereof.
 14. The optical device of claim 1 wherein:aportion of the optically nonlinear element between the optical input andthe optical output is divided into two arms of equal physical length;and an electrode is included in close proximity to one of said arms foraltering the optical path length of said arm.
 15. The optical device ofclaim 1 further comprising:means for changing the phase of opticalenergy applied to the optical input and transmitted from the opticaloutput comprising means for applying an electric field through theoptically nonlinear element.
 16. An optical device comprising an opticaltransmission element;at least part of the optical transmission elementcomprising an optically nonlinear element; the optically nonlinearelement comprising a triazine polymer containing a covalently bondedoptically nonlinear dye moiety; means for controlling the nature oflight transmitted by the optical transmission element comprising meansfor controllably applying a voltage to the triazine polymer.
 17. Thedevice of claim 16 wherein:the triazine polymer is made from a triazineprecursor; the triazine precursor isp-(N,N-bis(4'-cyanatobenzyl)amino)-p'-(2,2-dicyanovinyl)azobenzene. 18.The device of claim 16 wherein:the dye moiety is dicyanovinylazo.